1. Field of the Invention
The present invention relates to flow improvement in the processing of compositions containing PVC in processing machinery with polymers based on alkyl methacrylate.
2. Discussion of the Background
While soft polyvinylchloride (PVC) was a readily processable plastic at an early time from its content of plasticizers, substantial obstacles stood in the wa of thermoplastic processing of pure, high molecular weight PVC, particularly because of its thermal instability and its rheologic properties. These could be overcome by adding various aids, so that a PVC suitable for processing always consists of a mixture of various components. The main purpose of lubricants, which are used as processing aids along with other additives, is to improve the flow of high-viscosity PVC melts when working and plasticizing them in the different processing machines. Lubricants decrease the friction of the PVC particles and thereby prevent the production of considerable heats of friction, as well as the possible higher polymer temperature from friction between the surface of the processing machinery and the melt. PVC degradation reactions that may be caused by mechanical damage from shear forces and overheating are thereby avoided or distinctly reduced.
For processing hard PVC into semifinished product and finished parts, high-polymer processing aids are used based on polymethyl methacrylate (PMMA), which in addition to shortening the plasticizing process, bring about an improvement of the rheologic properties in the thermoplastic state. In contrast to other additives that affect the rheology, such as the plasticizers and lubricants already mentioned with molecular weights in g/mol of several hundred to about 10.sup.3, the molecular weights Mw of the high-polymer processing aids based on PMMA are between 1.2.times.10.sup.5 and 2.5.times. 10.sup.6 g/mol (Kunststoff Handbuch) 2/1, Polyvinyl chloride, 2nd edition, I986, Carl Hanser, Verlag Munich, Vienna, pages 595 to 597).
Molding compositions based on vinyl chloride polymers are disclosed by DE-A 35 42 469 in which the overall properties, especially the processing properties, can be distinctly improved by adding thermoplastic polymers of styrene, .alpha.-methylstyrene, p-methylstyrene, vinyltoluene, (meth)acrylonitrile, methyl methacrylate, or mixtures of them, and which have an intrinsic viscosity of 2 to 15 ml/g (measured in dimethylformamide at 25.degree. C.), and a sulfur content of 1.15 to 3.95 wt.%.
PVC can also be blended with other thermoplastics and elastomers to produce modified PVC, with blends being obtained that have technically valuable properties, such as PVC plastics with increased impact strength or better heat resistance.
Examples of structural materials, for example for manufacturing cases, are PVC/ABS blends in which the impact strength of the PVC is substantially improved by admixing the acrylonitrile-butadiene-styrene rubber (ABS). PVC and ABS are two incompatible polymers , i.e., blends of these two polymers are two-phased. Because of the incompatibility of the two types of polymer, large, macroscopic regions of inhomogeneity occur during mechanical processing of PVC-ABS blends, for example during the injection molding of large parts, which cause a high rejection rate of the processed polymer blend. The inhomogeneities are visible as streaks, splinters, scales, and separations into differently colored areas in the processed parts. These inhomogeneities of injection molded articles are attributable to inhomogeneities in the flow of the molten blend in the processing machine.
Therefore, there is the problem of preparing mixtures of PVC and other thermoplastically processable polymers such as PVC/ABS mixtures, which are processed mechanically to incompatible blends such as PVC/ABS blends so that finished parts with no macroscopic regions of inhomogeneity can be obtained reliably.
It has been found that the mechanical processing of incompatible polymer mixtures containing PVC into corresponding plastic blend articles with no macroscopic regions of inhomogeneity can be accomplished reliably with polymers based on alkyl methacrylate, which are copolymers of methyl methacrylate in particular, and have molecular weights Mw in the range of 2,000 to .ltoreq.20,000.